1. Field of the Invention
The present invention relates to edge-reflection surface acoustic wave filters using surface acoustic waves of the SH (shear horizontal) type such as BGS (Bleustein-Gulyaev-Shimizu) waves and Love waves, and more specifically, the present invention relates to an edge-reflection surface acoustic wave filter in which the structure of electrode fingers near reflection edges of interdigital transducers (hereinafter abbreviated as IDTs) is significantly improved.
2. Description of the Related Art
Edge-reflection surface acoustic wave filters have been used, for example, in bandpass filters for communications apparatuses. FIG. 12 is a schematic plan view showing an example of an edge-reflection surface acoustic wave filter according to the related art. The edge-reflection surface acoustic wave filter 101 includes a rectangular plate-type piezoelectric substrate 102. The piezoelectric substrate 102 has a first edge 102a and a second edge 102b disposed opposite to each other. On a top surface of the piezoelectric substrate 102, IDTs 103 and 104 are provided. The IDTs 103 and 104 have a pair of comb electrodes 103a and 103b and a pair of comb electrodes 104a and 104b, respectively. The comb electrodes 103a, 103b, 104a, and 104b have a plurality of electrode fingers 103a1, a plurality of electrode fingers 103b, a plurality of electrode fingers 104a1, and a plurality of electrode fingers 104b1, respectively. The electrode fingers 103a1 and the electrode fingers 103b1 are interdigitated with each other. Similarly, the electrode fingers 104a1 and the electrode fingers 104b1 are interdigitated with each other.
The electrode fingers 103a1 to 104b1 extend in a direction that is parallel to the edges 102a and 102b. 
In the edge-reflection surface acoustic wave filter 101, when an input voltage is applied to the IDT 103, a surface acoustic wave of the SH type is excited and reflected at the edges 102a and 102b. The surface acoustic wave reflected between the edges 102a and 102b generates a standing wave, and the resonance characteristics associated with the standing wave appear in the output from the IDT 104.
In the edge-reflection surface acoustic wave filter 101, of the electrode fingers 103a1 to 104b1, electrode fingers 103b1x and 104a1x that are located at an outermost position in the direction of propagation of surface acoustic waves (refer to FIG. 12) are arranged such that the outer edges thereof are aligned along the edges 102a and 102b. This is because the electrode fingers 103b1x and 104a1x are formed by cutting a mother piezoelectric substrate to form the edges 102a and 102b after a plurality of IDTs has been formed on the mother piezoelectric substrate.
However, the electrode fingers 103b, and 104a, adjacent to the reflection edges 102a and 102b are susceptible to damage in the cutting process, inevitably causing variation in filter characteristics.
With regard to the type of edge-reflection surface acoustic wave resonator using only one IDT, a method of reducing variation in characteristics due to damage to electrode fingers adjacent to reflection edges has been proposed. More specifically, Japanese Unexamined Patent Application Publication No. 60-41809 discloses a method of forming reflection edges at positions that are spaced by a distance that is equal to an integer multiple of λ/2 (λ is the wavelength of surface acoustic wave) from the centers of the outermost electrode fingers in the direction of propagation of surface acoustic waves. According to the disclosure, because the reflection edges are located at positions that are spaced by a distance that is equal to an integer multiple of λ/2 from the centers of the outermost electrode fingers in the direction of propagation of surface acoustic waves, the electrode fingers are not susceptible to damage in the cutting process for forming the reflection edges, whereby variations in resonance characteristics are reduced.
The method according to the related art reduces variations in resonance characteristics of an edge-reflection surface acoustic wave resonator. However, when the method is applied to an edge-reflection surface acoustic 14 wave filter, because the positions of the reflection edges are altered, filter characteristics become approximate to those in a case where the number of electrode fingers of IDT is increased. For example, if two IDTs having ten pairs of electrode fingers are disposed at an IDT interval of 1λ, the distance between the centers of the outermost electrode fingers on both sides is 21 λ. If reflection edges are disposed at a distance of 2×λ/2 (x/2 multiplied by an integer n, now assuming n=2) from the centers of the outermost electrode fingers of the IDTs, the distance between the reflection edges is 23λ. The filter characteristics in this case are approximate to those in a case where two IDTs having eleven pairs of electrode fingers are disposed at an IDT interval of 1k and reflection edges are disposed at the centers of the outermost electrode fingers. As a result, the bandwidth is reduced and the desired filter characteristics are not achieved.